1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine which uses alcohol blended gasoline for fuel and is operable at various blend ratios of alcohol to gasoline.
2. Description of the Prior Art
Though alcohol is a prospective future fuel for vehicles, a flexible fuel vehicle using alcohol blended gasoline and operable at various blend ratios of alcohol to gasoline has been developed as an actual solution at present. For example, Japanese Patent Publications SHO 56-66424 and SHO 57-76231 disclose such engines using alcohol blended gasoline as a fuel.
As a measure to counter environmental pollution due to NOx included in exhaust gas of vehicle engines, a catalyst constructed of zeolite carrying transition metals and capable of reducing NOx under an oxidizing gas condition in the presence of hydrocarbons also is now developed. However, the conventional zeolite catalyst has insufficient heat resistance and durability. To improve durability, Japanese Patent Publication HEI 1-130735 proposes to provide a fixed engine operating range in which degradation of the zeolite catalyst is prevented by shielding the catalyst from the exhaust gas.
When investigating the feasibility of installing such a zeolite catalyst in the exhaust line of an internal combustion engine operated on an alcohol blended gasoline fuel and to counter degradation of the catalyst by providing a fixed operational range to prevent degradation of the catalyst, it is difficult to achieve both high NOx reduction due to the catalyst and sufficient durability of the catalyst.
More particularly, the NOx reduction characteristic of the zeolite catalyst is strongly affected by the kinds of hydrocarbons included in the exhaust gas, and these are a function of the concentration of alcohol in the alcohol blended gasoline. For example, methyl alcohol, which has a single carbon atom (i.e. its carbon number is 1) has little effect in causing the catalyst to reduce NOx; so a high concentration of methyl alcohol in the fuel significantly decreases the NOx reduction rate of the catalyst. Therefore, if a fixed degradation prevention operating range is established so as to effectively utilize the catalyst on the basis of a fuel with low concentration of alcohol, little NOx will be reduced when a fuel with high concentration of alcohol is used. If the exhaust gas continues to flow through the catalyst despite the catalyst having little NOx reducing effect, the catalyst will uselessly be degraded. On the other hand, if a fixed degradation prevention operating range is established so as to effectively prevent the degradation of the catalyst on the basis of a fuel with high concentration of alcohol, the catalyst will be shielded from the exhaust gas at medium engine speeds and medium engine loads, even when a fuel with low concentration of alcohol is used. Therefore, the catalyst will not be effectively utilized, and a relatively great amount of NOx will be exhausted into the atmosphere at medium engine speeds and medium engine loads.